index-format: update preamble to cache tree extension

I had difficulty in my efforts to learn about the cache tree extension
based on the documentation and code because I had an incorrect
assumption about how it behaved. This might be due to some ambiguity in
the documentation, so this change modifies the beginning of the cache
tree format by expanding the description of the feature.

My hope is that this documentation clarifies a few things:

1. There is an in-memory recursive tree structure that is constructed
   from the extension data. This structure has a few differences, such
   as where the name is stored.

2. What does it mean for an entry to be invalid?

3. When exactly are "new" trees created?

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/index-format.txt

@@ -138,12 +138,33 @@ Git index format
 
 === Cache tree
 
-  Cache tree extension contains pre-computed hashes for trees that can
-  be derived from the index. It helps speed up tree object generation
-  from index for a new commit.
+  Since the index does not record entries for directories, the cache
+  entries cannot describe tree objects that already exist in the object
+  database for regions of the index that are unchanged from an existing
+  commit. The cache tree extension stores a recursive tree structure that
+  describes the trees that already exist and completely match sections of
+  the cache entries. This speeds up tree object generation from the index
+  for a new commit by only computing the trees that are "new" to that
+  commit. It also assists when comparing the index to another tree, such
+  as `HEAD^{tree}`, since sections of the index can be skipped when a tree
+  comparison demonstrates equality.
 
-  When a path is updated in index, the path must be invalidated and
-  removed from tree cache.
+  The recursive tree structure uses nodes that store a number of cache
+  entries, a list of subnodes, and an object ID (OID). The OID references
+  the existing tree for that node, if it is known to exist. The subnodes
+  correspond to subdirectories that themselves have cache tree nodes. The
+  number of cache entries corresponds to the number of cache entries in
+  the index that describe paths within that tree's directory.
+
+  The extension tracks the full directory structure in the cache tree
+  extension, but this is generally smaller than the full cache entry list.
+
+  When a path is updated in index, Git invalidates all nodes of the
+  recursive cache tree corresponding to the parent directories of that
+  path. We store these tree nodes as being "invalid" by using "-1" as the
+  number of cache entries. Invalid nodes still store a span of index
+  entries, allowing Git to focus its efforts when reconstructing a full
+  cache tree.
 
   The signature for this extension is { 'T', 'R', 'E', 'E' }.